Various conditions, e.g., age-related macular degeneration, retinitis pigmentosa, etc., exist which effect the functioning of photoreceptors (i.e., rods and cones) on a patient's retina and thus eliminate or severely degrade the patient's vision. To overcome these conditions, it has been proposed to directly stimulate the visual cortex or to implant a retinal prosthesis to stimulate neural pathways within a patient's retina. Stimulation of the visual cortex is described by Bindley G, Lewin W. “The sensations produced by electrical stimulation of the visual cortex,” J. Physiol (London) 1968:196:479-493. Apparatus for stimulation of the retina is described in U.S. Pat. No. 4,628,933 issued to Michelson on Dec. 16, 1986 for “Method And Apparatus For Visual Prosthesis,” and in U.S. Pat. No. 5,935,155 issued to Humayun et al on Aug. 10, 1999 for “Visual Prosthesis And Method Of Using Same.” The '933 patent and the '155 patents are herein incorporated by reference.
The '155 patent describes an electrode array adapted to be implanted on the retina, covering the fovea. Several methods of attaching the electrode array to the retina are described in the '155 patent. As is obvious from the methods of attaching the electrode array, the placement of the electrode array is likely to be in-exact. For example, the array may be translated and/or rotated relative to an ideal position. Additionally, nerves in the retina are not uniformly spaced, particularly when comparing the retina as a whole, and the macula.
Either translation or rotation of the electrode array, relative to ideal placement, may result a false perception of an object's location by a patient. Also, the perception of the size of an object associated with the stimulation may vary depending on which nerves are being stimulated. For example, stimulation of nerves within the macula may produce a different spatial perception than the same pattern of stimulation of nerves in the periphery around the macula. As a result, some objects may be perceived to be larger or smaller than they are, and the proportions of an object may be warped.
Accordingly, there is a need for methods and apparatus for adjusting the mapping of a pixilated image onto electrodes used for stimulation.